On a belt-driven air handling unit fan, the slide base is the part that determines both the position of the motor and the belt tension. If the new motor's foot holes do not fit the slide base, if the shaft height is different or if there is no tensioning margin left on the base, the motor will not run properly even if its kW rating and speed are correct. The size and condition of the slide base should therefore be checked before the motor.

DRG-112M-4 4 kW 1500 rpm cast iron frame DRG three-phase motor – air handling unit motor slide base

What problems does the slide base prevent?

The slide base allows the motor to be moved along the belt direction so that tension can be adjusted, and lets the motor be brought closer for a belt change without being removed. A loose belt causes slipping, heating and loss of airflow; an over-tightened belt places excessive radial load on the motor's drive-end bearing and on the fan bearings. Because the AHU section is cramped, when the slide base is inadequate service staff often make do by fixing the motor at an angle; this upsets pulley alignment and wears the belt along its edge.

Matching the new motor to the slide base

The IEC frame size determines the distance between the foot holes and the shaft height. If the old motor has a different frame size or is a non-standard product, the holes may not line up. When the frame size changes, the shaft height changes too; if the pulleys are not in the same plane, the belt runs skewed.

  • Measure the IEC frame size of the old motor (e.g. 112M) and its foot hole spacing.
  • Check how much of the slide base's forward–backward adjustment travel has been used.
  • Check whether the slide base bolts and tensioning screw are rusted or stripped.
  • If the anti-vibration mounts have hardened, consider renewing them along with the replacement.

Alignment and vibration

Once the motor is fixed to the slide base, the two pulleys must be in the same plane and parallel. In an AHU section, misalignment is felt as a hum transmitted into the ductwork and as vibrating filters. If the shaft diameter changes, the pulley hub must be renewed as well; making do with a bushing when the old and new motor shafts differ is not a permanent solution.

Details that also cover inverters and maintenance can be found on the air handling unit fan motors page. To compare frame sizes and shaft dimensions, you can use the model list in the shop.